WHERE子句
当我们需要根据指定条件从表中查询数据时,就可以在SELECT语句中添加WHERE子句,从而过滤掉我们不需要数据。WHERE子句构成一个行选择表达式,用于指定条件而获取的数据,如果给定的条件满足,才返回从表中的具体数值。
语法格式
SELECT
{ * | [column, ...] }
[ FROM from_item [, ...] ]
[ WHERE condition ];
参数说明
WHERE子句
WHERE子句构成一个行选择表达式,用来缩小SELECT查询的范围。在WHERE子句中可以使用比较运算符或逻辑运算符,例如 >, <, =, LIKE, NOT 等等。
condition
condition是返回值为布尔型的任意表达式,任何不满足该条件的行都不会被检索。
示例
使用逻辑运算符读取表customer_t1。
AND
找到c_customer_id为hello且c_customer_sk为3869的行。
openGauss=# SELECT * FROM customer_t1 WHERE c_customer_sk = 3869 AND c_customer_id = 'hello'; c_customer_sk | c_customer_id | c_first_name | c_last_name | amount ---------------+---------------+--------------+-------------+-------- 3869 | hello | Grace | | 1000 3869 | hello | Grace | | 1000 3869 | hello | | | 3869 | hello | | | (4 rows)
OR
找到c_customer_sk大于6985或者c_customer_id为hello的行。
openGauss=# SELECT * FROM customer_t1 WHERE c_customer_sk > 6985 OR c_customer_id = 'hello'; c_customer_sk | c_customer_id | c_first_name | c_last_name | amount ---------------+---------------+--------------+-------------+-------- 3869 | hello | Grace | | 1000 3869 | hello | Grace | | 1000 3869 | hello | | | 3869 | hello | | | 9976 | world | James | | 5000 (5 rows)
NOT NULL
找到customer_t1中c_last_name字段不为空的行。
openGauss=# SELECT * FROM customer_t1 WHERE c_last_name IS NOT NULL; c_customer_sk | c_customer_id | c_first_name | c_last_name | amount ---------------+---------------+--------------+-------------+-------- (0 rows)
BETWEEN
找到c_customer_sk在4000和9000之间的行。
openGauss=# SELECT * FROM customer_t1 WHERE c_customer_sk BETWEEN 4000 AND 9000; c_customer_sk | c_customer_id | c_first_name | c_last_name | amount ---------------+---------------+--------------+-------------+-------- 6985 | maps | Joes | | 2200 4421 | Admin | Local | | 3000 (2 rows)
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